Vehicles today use, for example, urea as reductant in SCR (selective catalytic reduction) systems which comprise an SCR catalyst, in which catalyst said reductant and NOx gas can react and be converted to nitrogen gas and water. Various types of reductants may be used in SCR systems. AdBlue is an example of a commonly used reductant.
One type of SCR system comprises a container for a reductant. The SCR system may also have a pump adapted to drawing said reductant from the container via a suction hose and to supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is adapted to injecting a necessary amount of reductant into the exhaust pipe upstream of the SCR catalyst according to operating routines stored in a control unit of the vehicle. To make it easier to regulate the pressure when no or only small amounts are being dosed, the system also comprises a return hose which runs back from a pressure side of the system to the container. This configuration makes it possible to cool the dosing unit by means of the reductant which, during cooling, flows from the container via the pump and the dosing unit and back to the container. The dosing unit is thus provided with active cooling. The return flow from the dosing valve to the container may be substantially constant.
During operation of the SCR system, thermal energy is stored in the exhaust system. This thermal energy may be transferred to, for example, the dosing unit.
Certain operating situations in which a cooling flow for the dosing unit is insufficient involve risk of the dosing unit being degraded functionally, becoming overheated and sustaining permanent damage or even completely disintegrating. Even temperatures which are not critical for hardware of the SCR system entail risk that the reductant therein might be adversely affected by too high temperatures, which might result in crystallisation of the reductant potentially leading to obstruction of, for example, the dosing unit.
WO 2009/020541 describes a system for emptying a reducing substance out of an injection system. The system has a heater connected to an injector with the object, for example, of vaporising the substance from the injector in order to avoid blockages in the injection system. However, the heater occupies space and its operation involves costs with regard to development, fitting, supervision during operation, and maintenance.
WO 2008/006840 describes a system for storing and supplying an additive substance in an exhaust system.
WO 2010/003424 A1 is mainly concerned with the temperature of the walls of the exhaust duct, which have been found, above a certain temperature, e.g. 200 degrees Celsius, to generate urea deposits in a nozzle from which the reducing agent is supplied to the exhaust duct. Above a certain temperature, an operating mode whereby reducing agent is from time to time used to blow the nozzle clear of deposits is therefore employed.
There is thus a need to improve current SCR systems in order to reduce or eliminate the above disadvantages.